Electronic flash unit having protective circuit for flash terminating switch

ABSTRACT

A semi-conductor switch interrupts the flashtube current, thereby terminating the flash when a blocking voltage is applied to its anode. The blocking voltage is derived from a capacitor. If the capacitor has not sufficiently recharged following a previous flash, the semi-conductor switch is not completely blocked causing it to burn out. Therefore, a transistor is provided whose emitter-collector circuit short-circuits the ignition capacitor when conductive. The base of the transistor is connected to the charging circuit for the capacitor furnishing the blocking voltage in such a manner that the transistor remains conductive until the capacitor charging current has decreased to a predetermined current.

United States Patent 1191 Pecher et al.

Helmut Schottner, Nurnberg, both of Germany [73] Assignee: Metz Apparatewerke Inh. Paul Metz, F urth/Bay, Germany [22] Filed: Oct. 17, 1972 21 Appl. No.: 298,277

[30] Foreign Application Priority Data Oct. 20, 1971' Germany 2152140 [52] US. Cl. 315/241 P, 315/156, 315/159,

250/205 [51] Int. Cl. H05b 41/40, H05b 41/32 [58] Field of Search 315/241 P, 241 R, 149,

156 References Cited UNITED STATES PATENTS 3,569.77) 971 Luursema ..315/24ll 1 June 4,1974

3.683.233 8/1972 Heintze 315/241 P Primary Examiner-Herman Karl Saalbach Assistant ExaminerRichard A. Rosenberger Attorney, Agent, or Firm-Michae1 S. Striker 5 7] ABSTRACT A semi-conductor switch interrupts the flashtube current, thereby terminating the flash when a blocking voltage is applied to its anode. The blocking voltage is derived from a capacitor. If the capacitor has not sufficiently recharged following a previousflash, the semi-conductor switch is not completely blocked causing it to burn out. Therefore, a transistor is provided whose emitter-collector circuit short-circuits the ignition capacitor when conductive. The base of the transistor is connected to the charging circuit for the capacitor furnishing the blocking voltage in such a manner that the transistor remains conductive until the capacitor charging current has decreased to a predetermined current.

11 Claims, 1 Drawing Figure ELECTRONIC FLASH UNIT HAVING PROTECTIVE CIRCUIT FOR FLASH TERMINATING SWITCH BACKGROUND OF THE INVENTION This invention relates to electronic flash units. In particular, it relates to such electronic flash units wherein the flash is automatically terminated when the quantity of light falling on the-film of an associated camera has reached a predetermined quantity. The present invention is particularly appropriate in the above-described electronic flash unit, when the termination of the flash is accomplished by use of a semi-conductor switch means having an anode-cathode circuit connected in series with the flashtube. Terminationof the flash is then achieved by applying to the anode of said semiconductor means a blocking voltage furnished by a terminating capacitor. The blocking voltage is applied in response to a signal generated by a light measuring circuit.

In the electronic flash unit as described above, the terminating capacitor must be re-charged after each flash termination. Of course it must be re-charged to a predetermined voltage which is the required blocking voltage for the semi-conductor switch. However, recharging of the terminating capacitor to such a predetermined voltage requires a predetermined time interval. If the flashtube were to be ignited prior to the elapse of this predetermined time interval, the terminating capacitor would not be fully charged. In this case, when the light measuring circuit furnishes the signal indicating that the flash is to be terminated, the insufficient charge on the terminating capacitor would cause a voltage less than the blocking voltage to be applied to the anode of the senii-conductor switch, thereby causing it to remain conductive. Under these conditions. namely when the semi-conductor switch is partially conductive (in the region between the blocked and the fully conductive state), maximum power is dissipated in said semi-conductor and the semi-conductor will probably be immediately destroyed.

The above-described conventional electronic flash units further have the disadvantage that, in general, they have an indicator lamp connected to the ignition circuit for the flash tube. Specifically, the indicator lamp glows whenthe capacitor furnishing the main flash energy is charge to a predetermined voltage. It can thus happen that after a relatively short flash this main capacitor is still sufficiently charged to cause ignition of the glow lamp, thereby indicating to the photographer that the flash unit is ready for operation prior to the time that the terminating capacitor has reached the voltage necessary for blocking the terminating semiconductor switch, Under these circumstances, the photograp'her may initiate a flash, thereby possibly destroying the semi-conductor terminating switch, or in any case. not achieving the correct illumination of the film.

SUMMARY OF THE INVENTION It is an object of the present invention to prevent the destruction of the semi-conductor switch and the possible early ignition of the glow lamp indicating circuit readiness for operation, as described above.

The present invention thus resides in an electronic flash unit having flashtube means for furnishing a light flash upon receipt of an ignition signal. The electronic flash unit further has ignition circuit means connected to said flashtube means for furnishing said ignition signal upon external activation. First switch means having at least a first electrode are connected to said flashtube means for terminating said light flash upon application of a first predetermined voltage to said first electrode. Terminating capacitor means are provided for furnishing a capacitor voltage. Applying means apply said capacitor voltage to said first electrode upon receipt ofa terminating signal. Light measuring means furnish said terminating signal. The improvement of the present invention comprises additional circuit means interconnected between said terminating capacitor means and said ignition circuit means for inhibiting the operation of said ignition circuit means until said capacitor voltage is equal to said first predetermined voltage.

It is thus seen that the ignition circuit remains inoperative until the proper blocking voltage for the first (terminating) switch means is available. I

In a preferred embodiment of the present invention, the ignition circuit meanscomprise an ignition capacitor which furnishes an ignition voltage when charged. The additional circuit means then comprise delay circuit means which delay the charging of said ignition capacitor means to said ignition voltage until the terminating capacitor means furnish a predetermined capacitor voltage corresponding to said first predetermined voltage. Specifically, the predetermined capacitor voltage may be slightly less than said first predetermined voltage.

ln a further preferred embodiment of the present invention the terminating capacitor has an associated charging circuit which furnishes a charging current thereto. The voltage across a portion of the charging circuit is applied to the base of a transistor whose emitter-collector circuit is connected to the ignition circuit and, specifically, short-circuits the ignition capacitor when conductive. Thus the ignition capacitor remains discharged until the transistor of the additional circuit means becomes non-conductive. The base of the transistor is connected to the charging circuit for the terminating capacitor in such a manner that the transistor becomes non-conductive when the capacitor is almost fully charged.

It is a particular advantage of the above-described arrangement that no photographer initiated flashtube ignition nor an accidental undesired ignition of the flashtube can take place until the terminatingcapacitor is charged to the proper blocking voltage. Further, if as in another preferred embodiment of the present invention, the indicator lamp indicating equipment readiness is connected in parallel with the emitter-collector circuit of the transistor of the additional circuit means, then no indication can occur that the equipment is ready until the terminating capacitor has been properly charged.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however,

.both as to its construction and its method of operation,

BRIEF DESCRIPTION OF THE DRAWINGS The single FIGURE is a schematic diagram showing a circuit of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will now be described with reference to the drawing.

The voltage source for the electronic flash unit is labelled G] in this FIGURE. It charges the main storage capacitor C l and the terminating capacitor C2 through a diode labelled D. The charging of terminating capacitor C2 is accomplished through a charging circuit comprising resistors RI, R2 and R3. Resistors R1, R2 and R3 thus constitute charging circuit means for terminating capacitor C2. More specifically, resistors R1 and R2 form voltage divider means having a voltage divider tap which is connected to the base of transistor labelled T,. Transistor T, is herein referred to as additional circuit means, and more specifically, as third switch means. Although it is illustrated here as being a transistor, it could readily be replaced by a thyristor or other switching element. The controlled circuit of the third switch means, namely the emitter-collector circuit of transistor T, is connected in parallel with second switch means labelled 8,, in the FIGURE. The switch means 5,, is the synchronous contact operated in conjunction with the camera release. Contact when closed, and the emitter-collector circuit of transistor T, are both connected in parallel with an ignition capacitor C3, thereby short-circuiting said ignition capacitor when in a conductive state. While so short-circuited, ignition capacitor C3 of course cannot charge. The voltage across resistor R2, which of course is the voltage at the base of transistor T,, is a maximum for maximum charging current through terminating capacitor C2. When this current has decreased sufficiently, that is when terminating capacitor C2 is almost fully charged, the voltage at the base of transistor T, becomes sufficiently small to block said transistor, thereby allowing capacitor C3 to charge.

Capacitor C3 has one terminal connected to ground through the primary winding of an ignition transformer Z, while its other terminal is connected to the common point of resistors R4 and R5. The other terminal of resistor R4 is connected to the cathode of the abovementioned diode D, while the other terminal of resistor R5 is connected to ground. The synchronous contact 8,, as well as the emitter-collector circuit of transistor T, are connected in parallel with resistor R5. Thus, when transistor T, becomes non-conductive and switch 8,, is open, capacitor C3 will charge through resistor R4 and the primary winding if ignition transformer Z to a voltage determined by the voltage divider ratio of resistors R4 and R5. An indicator lamp S also connected in parallel with resistor R5 will light when the voltage across capacitor C3 is almost equal to the ignition voltage. Thus, when indicator lamp S lights, not only is the ignition capacitor sufficient charged, but also the terminating capacitor is fully charged.

If contact 8,, is now closed, capacitor C3 discharges rapidly. furnishing rapidly increasing current to the primary winding of the ignition transformer. This induces a pulse in the secondary winding of the ignition transformer which is connected both to the ignition electrode of flashtube labelled R, in the FIGURE and to the control electrode of the first switch means, here a thyristor labelled T Thus the discharge of capacitor C3 furnishes an ignition signal upon receipt of which the flashtube ignites, thyristor T,, becomes conductive, (first state of thyristor T and the flash is created.

During the light flash a light measuring circuit labelled LM in this FIGURE is operative. This light measuring circuit may, for example, be a circuit as shown in the US. Pat. No. 3,033,988 to H. E. Edgerton. Specifically, such a light measuring circuit is shown in FIG. 1 of the Edgerton patent and comprises a light sensitive tube 61 and a capacitor SI, the charge on the capacitor increasing in proportion to the current through the light sensitive tube. The light sensitive tube is so mounted as to receive the light from the flashtube. In the case of a flashtube associated with the camera, the light sensitive tube or element would of course be mounted to receive a portion of the light received by the film. Line 53 of FIG. 1 of the Edgerton patent would be connected to the gate of thyristor T In any event, a light measuring circuit is operative during the flash and furnishes the terminating signal when the soreceived quantity of light has reached a predetermined quantity. This light measuring circuit constitutes means furnishing a terminating signal. When the terminating signal is received, that is when the voltage furnished by the light measuring circuit has reached a voltage corresponding to the desired quantity of light having im pinged upon the photosensitive element, thyristor T, fires. Reference to the FIGURE shows that thyristor T, has a cathode connected to ground and an anode connected to one terminal of capacitor C2 whose other terminal is connected to the anode of thyristor T As stated above, capacitor C2 was previously charged to substantially the operating voltage furnished by source G1, the positive voltage appearing at the anode of thyristor T Thus when thyristor T fires and'one terminal of capacitor C2 is thus directly connected to ground potential, a negative bias voltage substantially equal to the operating voltage appears at the anode of thyristor T thereby blocking the same. (Thyristor T thus constitutes means for applying the voltage across capacitor C2 to the first electrode (anode) of thyristor T The blocked state of thyristor T is herein referred to as its second state.) The charge current from capacitor Cl which prior to this blocking flow through thyristor T is now used to charge capacitor C2 in the opposite direction. The bias voltage applied to the anode of thyristor T thus decreases. It reaches a zero value and then continues in the positve direction until a value is reached which is approximately the same as the operating voltage. When the charging current is approximately zero, thyristor T blocks and flashtube R extinguishes. Capacitor C2 is now recharged through its charging circuit comprising resistors R1, R2 and R3 in the original direction. The voltage appearing across resistor R2 during this charging of capacitor C2 causes transistor T, to be conductive, thereby short-circuiting capacitor C3 which is the ignition capacitor. Until capacitor C2 is almost fully charged, transistor T, continues to short circuit the ignition capacitor, thereby preventing operation of the ignition circuit. Also, since it is connected in parallel with the emittercollector circuit of transistor T,, the indicator lamp S cannot light.

Thus, during the time that capacitor C2 charges any closing of contact Sk, whether accidental or intended,

remains ineffective. Only when capacitor C2 has reached a charge whereby the voltage across said capacitor would be sufficient to block thyristor T is transistors T, blocked allowing capacitor C3 to charge. Only when the charge of capacitor C3 is sufficiently high is the circuit again ready for operation, which will also be indicated by the lighting of lamp S.

While the invention has been illustrated and described as embodied in a flashtube unit using a series switch for terminating the flash and using specific types of switches such as thyristors and transistors, it is not intended to be limited to the details shown, since circuit and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adapt it for various applications without omitting features that, from the standpoint of prior art fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. in an electronic flash unit, in combination, a source of electrical energy; flash generating means connected to said source of electrical energy for furnishing a light flash upon receipt of an ignition signal; ignition circuit means connected to said flash generating means for furnishing said ignition signal upon external activation; first switch means connected to said flash generating means, said first switch means having at least a first electrode and having a first state during said flash, for switching to a second state blocking said flash upon application of a predetermined voltage to said first electrode; terminating capacitor means connected to said source of electrical energy in such a manner that the voltage across said terminating capacitor means varies as a predetermined function of time following termination of a flash; means for applying said voltage across said terminating capacitor means to said first electrode upon receipt of a terminating signal; means for furnishing said terminating signal at the end of a desired exposure time; and additional circuit means interconnected between said terminating capacitor means and said ignition circuit means for inhibiting the operation of said ignition circuit means until said voltage across said terminating capacitor means is substantially equal to said predetermined voltage.

2. An electronic flash unit as set forth in claim 1, wherein said first switch means is connected in series with said flash generating means.

3. An electronic flash unit as set forth in claim 2, wherein said first switch means comprises a semiconductor switch means having an anode-cathode circuit connected in series with said flash generating means and having a control electrode connected to said ignition circuit means.

4. An electronic flash unit as set forth in claim 3, wherein said first electrode of said first switch means is said anode and wherein said predetermined voltage is the voltage for blocking said semi-conductor switch means.

5. An electronic flash unit as set forth in claim 4, wherein said ignition circuit means comprise ignition capacitor means for furnishing an ignition voltage when charged; and wherein said additional circuit means comprise means for preventing the charging of said ignition capacitor means to said ignition voltage until said terminating capacitor means is charged to a predetermined capacitor voltage corresponding to said predetermined voltage.

6. An electronic flash unit as set forth in claim 5, further comprising charging circuit means connected to said terminating capacitor means for furnishing a charging current thereto; and wherein said ignition circuit means comprise second switch means responsive to said external activation, for initiating said light flash upon activation thereof.

7. An electronic flash unit as set forth in claim 6, wherein said additional circuit means comprise third switch means connected to said second switch means, said third switch means having a conductive state, for short circuiting said second switch means when in said conductive state.

8. An electronic flash unit as set forth in claim 7, wherein said third switch means comprise additional semi-conductor switch means having a controlled circuit connected in parallel with said second switch means and having a control electrode connected to said charging circuit means.

9. An electronic flash unit as set forth in claim 8, wherein said additional semi-conductor switch means comprise transistor means, wherein said controlled circuit is the emitter-collector circuit of said transistor means and wherein said control electrode is the base of said transistor means; further comprising connecting means for connecting said base of said transistor means to said charging circuit in such a manner that the voltage at said base varies as a function of said charging current of said terminating capacitor means.

10. An electronic flash unit as set forth in claim 9, wherein said charging circuit means comprise voltage divider means having a voltage divider tap; and wherein said connecting means connects said base to said voltage divider tap.

11. An electronic flash unit as set forth in claim 7, further comprising indicator lamp means connected in parallel with said second and third switch means. 

1. In an electronic flash unit, in combination, a source of electrical energy; flash generating means connected to said source of electrical energy for furnishing a light flash upon receipt of an ignition signal; ignition circuit means connected to said flash generating means for furnishing said ignition signal upon external activation; first switch means connected to said flash generating means, said first switch means having at least a first electrode and having a first state during said flash, for switching to a second state blocking said flash upon application of a predetermined voltage to said first electrode; terminating capacitor means connected to said source of electrical energy in such a manner that the voltage across said terminating capacitor means varies as a predetermined function of time following termination of a flash; means for applying said voltage across said terminating capacitor means to said first electrode upon receipt of a terminating signal; means for furnishing said terminating signal at the end of a desired exposure time; and additional circuit means interconnected between said terminating capacitor means and said ignition circuit means for inhibiting the operation of said ignition circuit means until said voltage across said terminating capacitor means is substantially equal to said predetermined voltage.
 2. An electronic flash unit as set forth in claim 1, wherein said first switch means is connected in series with said flash generating means.
 3. An electronic flash unit as set forth in claim 2, wherein said first switch means comprises a semi-conductor switch means having an anode-cathode circuit connected in series with said flash generating means and having a control electrode connected to said ignition circuit means.
 4. An electronic flash unit as set forth in claim 3, wherein said first electrode of said first switch means is said anode and wherein said predetermined voltage is the voltage for blocking said semi-conductor switch means.
 5. An electronic flash unit as set forth in claim 4, wherein said ignition circuit means comprise ignition capacitor means for furnishing an ignition voltage when charged; and wherein said additional circuit means comprise means for preventing the charging of said ignition capacitor means to said ignition voltage until said terminating capacitor means is charged to a predetermined capacitor voltage corresponding to said predetermined voltage.
 6. An electronic flash unit as set forth in claim 5, further comprising charging circuit means connected to said terminating capacitor means for furnishing a charging current thereto; and wherein said ignition circuit means comprise second switch means responsive to said external activation, for initiating said light flash upon activation thereof.
 7. An electronic flash unit as set forth in claim 6, wherein said additional circuit means comprise third switch means connected to said second switch means, said third switch means having a conductive state, for short circuiting said second switch means when in said conductive state.
 8. An electronic flash unit as set forth in claim 7, wherein said third switch means comprise additional semi-conductor switch means having a controlled circuit conNected in parallel with said second switch means and having a control electrode connected to said charging circuit means.
 9. An electronic flash unit as set forth in claim 8, wherein said additional semi-conductor switch means comprise transistor means, wherein said controlled circuit is the emitter-collector circuit of said transistor means and wherein said control electrode is the base of said transistor means; further comprising connecting means for connecting said base of said transistor means to said charging circuit in such a manner that the voltage at said base varies as a function of said charging current of said terminating capacitor means.
 10. An electronic flash unit as set forth in claim 9, wherein said charging circuit means comprise voltage divider means having a voltage divider tap; and wherein said connecting means connects said base to said voltage divider tap.
 11. An electronic flash unit as set forth in claim 7, further comprising indicator lamp means connected in parallel with said second and third switch means. 